Myeloid derived suppressor cells (MDSC) represent one of the most powerful mechanisms used by tumors to evade the immune response and create T cell tolerance. During the previous cycle of this grant, we studied murine and human MDSC. Our data demonstrated that MDSC expressing arginase I deplete L-arginine from the microenvironment, triggering activation of the GCN2 kinase pathway which eventually leads to an arrest in T cell cycle progression, inhibition of IFN? production, and blocking of signaling through the T cell receptor. Other researchers have recently described three additional mechanisms by which MDSC may suppress T cell function, namely the production of peroxynitrites, the production of H2O2, and the induction of regulatory T cells. Therefore, it is essential to identify the primary mechanism by which MDSC induce T cell tolerance in vivo so targeted therapies that block MDSC function can be designed and tested. To achieve this goal, we developed a new strain of conditional arginase I knock-out mice (full knock-outs die soon after birth) in which arginase I is deleted only in the CD11b+ myeloid cells. We have also established additional murine knock-out colonies to study and compare the proposed mechanisms for the induction of T cell tolerance by MDSC. Our preliminary data continue to support the hypothesis that MDSC expressing arginase I deplete L-Arginine in the tumor microenvironment and thereby cause T cell tolerance. Inhibiting arginase I will result in the development of a protective anti-tumor T cell response. In addition, our data shows that human tumors activate and promote the survival of MDSC, allowing them to accumulate in cancer patients. Inhibiting these tumor derived signals may also represent a new therapeutic approach to blocking MDSC. To definitively identify the mechanisms by which MDSC induce T cell tolerance and determine how human tumors activate MDSC, we propose the following Specific Aims. 1. Test the hypothesis that the primary mechanism for the induction of T cell tolerance by MDSC is the production of Arginase I. 2. Determine the mechanism(s) by which MDSC induce T cell tolerance in vivo and determine its effect on the anti-tumor response. 3. Determine the mechanisms by which tumors promote the survival and inhibit apoptosis of human MDSC. 4. Determine which is the predominant mechanism for the induction of T cell tolerance by human MDSC.